This invention relates to caustic solutions having a greatly reduced corrosive effect on nickel-containing metal surfaces. More particularly, it relates to the use of hydrazine or its salts or derivatives as corrosion inhibitors in caustic soda or caustic potash solutions. Still more particularly, it relates to an improvement in the manufacture of concentrated caustic soda solutions wherein hydrazine or its salts or derivatives are added in an extremely small but effective amount to the caustic soda solution prior to its final dehydration such that the otherwise severe corrosion of the nickel evaporation equipment in which such dehydration is normally conducted is greatly reduced or eliminated.
With an increased emphasis on energy conservation, new plants for the manufacture of caustic soda or potash are being designed with an increasing number of evaporation stages wherein relatively dilute caustic alkali solutions are dehydrated to produce commercially desired solutions containing alkali metal hydroxide concentrations of at least 40 percent. As a consequence, higher temperatures are being employed in the final evaporation stages of such new plants than in plants based on earlier designs and more serious corrosion of the customarily employed nickel equipment consequently occurs.
Corrosion control has been equated in the past with the complete elimination of oxidants, especially the chlorates normally present in caustic soda coming from a diaphragm cell. A wide variety of methods have been proposed for this purpose. See, for instance, CA79:138308u; CA79:80874g; CA78:161639r; CA79:23333k; CA78:18401b; CA67:557888b; CA63:9519f; CA61:6655c; CA57:P9458i; U.S. Pat. No. 3,042,491; U.S. Pat. No. 2,889,204; U.S. Pat. No. 2,823,177; U.S. Pat. No. 2,790,707; U.S. Pat. No. 2,735,730; CA50:9941b; U.S. Pat. No. 2,610,105; U.S. Pat. No. 2,562,169; Brit. Pat. No. 642,946 (Sept. 13, 1950); Brit. Pat. No. 597,564 (Jan. 28, 1948); U.S. Pat. No. 2,415,798; U.S. Pat. No. 2,404,453; U.S. Pat. No. 2,403,789; German Pat. No. 708,059 (June 5, 1941); U.S. Pat. No. 2,258,545; French Pat. No. 799,632 (June 16, 1936).
More recently, U.S. Pat. No. 3,325,251 has disclosed a method of reducing oxidative attack on nickel-containing metal surfaces during the dehydration of aqueous caustic solutions which involves the addition of formic acid, oxalic acid, or their sodium or potassium salts, in an amount of 50 to 500 ppm based on the NaOH or KOH content of the solution. However, this patent specifically suggests that larger amounts of the reducing agent may be required when the caustic liquor to be evaporated contains considerable quantities of an oxidizing substance such as a chlorate and/or oxygen.
Several of these prior methods have been or are being used commercially, but the use of sodium sulfite to control oxidizing agents in caustic liquors is probably mostly widely practiced because of its cost advantage over many or most of the other methods previously suggested for oxidant removal. There are, however, also significant disadvantages related to the use of sodium sulfite as a corrosion inhibitor in concentrated caustic alkali metal hydroxide solutions, because of its limited solubility (at 100.degree. C., the solubility of sodium sulfite in 42% sodium hydroxide saturated with sodium chloride was found to be only 0.114 wt %) and because of resultant increased levels of sodium sulfate in both the caustic soda or potash product and in recycled brine from such dehydration processes. This is then disadvantageous to brine recycled to electrolytic cell processes.
In connection with a different art and a different problem, it may be mentioned that U.S. Pat. No. 3,620,777 suggests the inclusion of 2 to 20 g/l of hydrazine hydrate or hydrazine salt, in a chromate coating solution, which coating is said to impart good corrosion resistance to zinc alloy surfaces such as galvanized steel. However, this patent emphasizes the need for keeping the pH value of the coating solution within the distinctly acid range of 0.8 to 3.5 and emphasizes that the existence of any matter, except zinc, that would cause the rise of the pH of the solution above 3.5 is not desirable and that the addition of an alkali metal ion should be avoided. This reference does not address the problem of corrosion of nickel-containing surfaces, and in fact that applicability of its teachings to an alkali metal containing system is expressly contraindicated.
It has now been discovered that hydrazine and its salts and derivatives when used in very small but effective proportions, such as 1,000 ppm or less, preferably 200 ppm or less and most preferably 40 ppm or less, serve as excellent corrosion inhibitors when caustic liquor is dehydrated or otherwise processed in equipment made of nickel or alloys composed prodominantly of nickel, and that the effectiveness of this new method of corrosion inhibition surprisingly does not seem to depend on the substantial elimination of oxidant impurities from the caustic liquor. In fact, the invention is effective even when the hydrazine inhibitor is added to the caustic solution in a proportion substantially smaller than that required to reduce all of the oxidizing agent present, e.g., chlorate to chloride. For instance, the addition of as little as 1% or less of the stoichiometric amount of hydrazine offers significant protection.
There is considerable literature on the use of hydrazine as a corrosion inhibitor for steam boilers, i.e., as a corrosion inhibitor for steel surfaces exposed to substantially neutral or acidic water. However, the use of hydrazine as a corrosion inhibitor for nickel or nickel alloys in contact with concentrated caustic liquors does not appear to have been proposed. In steam boiler applications, hydrazine is used primarily as a molecular oxygen scavenger. As pointed out in the trade literature, the key mechanism for hydrazine as a corrosion inhibitor with steel boilers is not merely its direct reaction with the dissolved oxygen in the feed water, but also, and importantly, the development of a dense, protective coating of magnetite, Fe.sub.3 O.sub.4, to which ferric oxide already formed on the steel surface is reduced by the reaction with hydrazine. See brochure 731-006R, entitled "SCAV-OX 35% Hydrazine Solution for Corrosion Protection In High, Medium and Low Pressure Boilers", published in 1978by Olin Chemicals, 120 Long Ridge Road, Stamford, Connecticut 06904.
Further, the use of alkyl hydrazines for the removal of free oxygen from liquids or gases at ambient or low temperature so as to avoid corrosion of boilers, ducts, pipes and the like has been proposed in U.S. Pat. No. 3,962,113, especially for water at a pH between about 3 and 8, i.e., essentially neutral or acid streams in which hydrazine itself is said to be relatively ineffective.
Hydrazine hydrate has also been proposed in the past as a reagent for the substantially complete decomposition of chlorate in aqueous alkali metal hydroxide. See CA84:137943p, Japan Kokai No. 73,134,996, Oct. 25, 1975. According to this proposal, hydrazine hydrate is added to the chlorate-containing caustic liquor, in the presence of iron as a catalyst, in an amount substantially greater than the stoichiometric amount required for the reduction of the chlorate to chloride. However, the cost of the proposed chlorate removal reagent is very high at the indicated concentrations. Furthermore, residence times required for the proposed chlorate removal are very long and the iron catalyst that is disclosed to be required in this prior proposal results in an undesirable contamination of the final caustic soda product.